1. Field of the Invention
This invention relates to an improved catalyst used in ethylene polymerization, and more particularly, to vanadium containing catalyst compositions having high activity and providing an improved process in the gas phase for producing polyethylenes having a desirable balance of properties useful in certain applications.
2. Description of the Prior Art
Polyethylenes having a relatively broad molecular weight distribution possess wide utility in the areas of wire and cable insulation, blow molding and pipe fabrication. Such broad molecular weight distribution polyethylenes, produced for example by the process disclosed in U.S. Pat. No. 3,324,095 (Carrick et al.), possess instability through the presence of a relatively high level of unsaturation. Relatively narrow molecular weight distribution polyethylenes can be made in the gas phase as described in for example U.S. Pat. No. 4,302,565 (Goeke et al.) and U.S. Pat. No. 4,302,566 (Karol et al.). Such gas phase polymerization procedures use catalysts with high activity to produce polyethylene low in unsaturation and possessing high utility in various film applications. Such polyethylenes, however, have limited utility in other areas requiring the properties of increased melt flow and extrudability as well as lower melt fracture found in broad molecular weight distribution polyethylenes. It would therefore be desirable to develop a high activity catalyst composition and process for producing a highly saturated, polyethylene in the gas phase having a broad molecular weight distribution.
Vanadium catalysts have been used to produce polyethylenes. U.S. Pat. No. 3,956,255 (Ort) discloses an ethylene polymerization catalyst composition consisting essentially of a vanadium compound combined with an alkylaluminum alkoxide and a trialkylaluminum. This catalytic composition is deposited on silica which has been reacted with an alkylaluminum alkoxide. Optionally, a halogenated alkane may be used as promoter.
U.S. Pat. No. 4,173,698 (Kanoh et al.) describes an ethylene polymerization catalyst which is the unsupported product of a vanadium trichloride-ether complex combined with an aluminum trihalide. An organoaluminum compound is used as cocatalyst.
U.S. Pat. No. 4,202,958 (Yamaguchi et al.) pertains to an ethylene polymerization process using an unsupported, solid catalyst obtained by reducing a vanadium halide with an organic aluminum compound in the presence of an ether compound. An organic aluminum cocatalyst is also provided.
U.S. Pat. No. 4,263,171 (Shida et al.) is representative of disclosures of olefin polymerization catalysts using magnesium containing compounds. The patent discloses a silica supported catalyst of a magnesium-aluminum alkyl complex and halogen-containing transition metal compound (including vanadium).
U.S. Pat. No. 4,294,947 (Doerk et al.) describes an ethylene/butene-1 copolymerization in a liquid butene-1 solvent using an unsupported catalyst comprising the reaction product of a vanadyl chloride and/or alkoxide with alkylaluminum chloride and organoaluminum cocatalysts.
Many of these references, representative of the prior art, are directed towards providing catalyst with high activity or towards select improvements in particular polymerization procedures or polyethylene properties. Because of the complexity, it has not been possible, prior to the present invention, to provide a commercially useful, optimized process in the gas phase using a vanadium catalyst to produce polyethylenes having a desirable balance of properties. In order to provide such a process, the catalyst must be one that can, simultaneously; provide a superior hydrogen response in order to achieve very low levels of unsaturation in the polyethylene; provide a high degree of productivity such that the catalyst residues in the polyethylene are so small, preferably less than about 10 ppm of vanadium, so as to allow them to remain therein thereby avoiding catalyst removal steps; provide a polyethylene which can be readily molded into a variety of molding applications, i.e. provide a polyethylene having relatively broad molecular weight distribution and wide melt index range; provide enhanced copolymerization of ethylene with other alpha olefins to enable a wide range in density control of the polyethylene as well as a broader range of comonomers which can be used; provide a polyethylene possessing a high bulk density in order to be commercially acceptable in handling and transportation costs; provide a polyethylene having a particle size which can be readily fluidized in a fluid bed reactor, and which is low in fines content; provide freely-flowing particulate polyethylene free of fused or chunk like agglomerations; preferably using the simplest catalyst composition possible consisting essentially of a minimum number of components.